Used car buyers tempted by diesels could have to do up to 56,373 miles before reaping the financial benefit of the fuel savings they believe are on offer.

The new research by used car price experts, WiseBuyer’s (www.wisebuyers.co.uk), disputes the idea that choosing a diesel automatically drives down the cost of motoring compared to a petrol alternative.

On average, the cost of buying a used diesel is between 8-12% higher than its petrol equivalent. And service costs for some diesel models are higher.

WiseBuyer’s data shows that a twelve-month-old Ford Focus 1.8 TDCi LX typically costs £1,150 more on the forecourt than its 1.6 LX petrol alternative. Although the former records an average 53.3mpg compared to 40.9mpg for the petrol version, the actual fuel cost saving is only 2.04 pence per mile travelled. So owners would need to complete a staggering 56,373 miles before fuel savings recoup the extra cost of buying a diesel.

A Volkswagen Lupo 1.4 TDi PD Sport, which costs just £350 more than its 1.4 Sport petrol cousin, needs 14,286 miles to be covered before you start benefiting from the more economical diesel engine.

Purchasing a seven-seater Ford Galaxy 1.9 TDi Zetec diesel would demand the owner completing 43,237 miles before matching the £1,950 cheaper 2.3 Zetec petrol equivalent.

Nic Barfield, editor of WiseBuyer’s, says: "For someone who only does 10,000 miles a year, a diesel may not be the best option. As the research shows, it could take up to five years of average motoring to start feeling the financial benefit of the fuel savings."